Here's a scene where my night fog worked out pretty well, with some volumetric lights:
http://www.renderosity.com/mod/gallery/index.php?image_id=1446146

I tried MetaClouds and didn't get very good results. What worked for me was to create a fog "terrain" for the close-up low fog, add a custom fog material (started with one of the default fuzzy glows). Then I mixed that with a very thin spectral cloud layer above it, by moving the whole scene, plus the ground, up to the cloud layer altitude. The volumetric spot lights also have that artificial smoke feature enabled and the function for that tweaked.